The information provided herein is intended solely to assist the understanding of the reader. None of the information provided nor references cited is admitted to be prior art to the present invention. Each of the references cited herein is incorporated herein by reference in its entirety.
Kinases are a large family of proteins that have now become firmly established as a major class of drug targets. The sequencing of the Human Genome has led to the identification of over 500 protein kinases encoded within it: the Human Kinome. These protein kinases have been grouped into a total of seven families, based on their structures. The 388 serine/threonine kinases fall into five families: AGC, CAMK, CMGC, CK1, and STE. The 90 tyrosine kinases fall into two families: the TK (tyrosine kinase) family of 58 kinases and the TKL (tyrosine kinase-like) family of 32 kinases. There are 40 atypical kinases that are highly structurally distinct from the remainder. The TK family is a source forvalidated drug targets, such as epidermal growth factor receptor (EGFR), vascular endothelial growth factor receptor (VEGFR), and platelet-derived growth factor receptor (PDGFR). Recent clinical data provided proof-of-concept for selective B-Raf inhibitors in the treatment of B-RafV600E mutant melanoma, which led to the approved clinical use of vemurafenib for the treatment of late stage metastatic melanoma.
The regulation of cancer cell growth is often modulated by multiple kinases acting both in independent pathways and in concerted signaling cascades. Thus, the Ras/Raf/MEK/ERK mitogen-activated protein kinase (MAPK) pathway mediates cellular responses to different growth signals and is frequently deregulated in cancer. The Raffamily consist of serine/threonine kinases A-Raf, B-Raf and C-Raf (Raf-1), which phosphorylate and activate MEK. Only B-Raf is frequently mutated in various cancers; and the most common B-Raf mutation involves a substitution of a glutamic acid residue to a valine moiety at codon 600 (BRAFV600E). Mutations in the B-Raf gene may lead to MAPK pathway amplification via constitutive activation of B-Raf kinase, even in the absence of any growth signals, resulting in increased malignancy.
A landmark study published in 2002 identified activating mutations in the B-Raf serine/threonine kinase gene BRAF in more than 50% of all melanomas. In addition to melanoma, activating BRAF mutations have also been identified in colorectal carcinomas (where it occurs in 40% of tumors with mismatch repair deficiency), 50% of thyroid papillary carcinomas and 30% of low-grade serous ovarian carcinomas. Although related isoforms of Raf, such as A-Raf and C-Raf, can activate other signaling pathways, it is believed that the transforming activity of B-Raf arises mainly through the stimulation of the MAPK pathway.
Melanomas are tumors derived from the transformation of melanocytes, a family of specialized pigment-producing cells. Melanomas are broadly divided into those arising from the skin (cutaneous), eyes (uveal) or mucous membranes (mucosal). Although melanoma is only the third most common form of skin cancer in people of European descent (after squamous cell carcinoma and basal cell carcinoma), it accounts for nearly all skin cancer deaths. Once disseminated to distant organs, melanoma becomes a considerable clinical problem associated with high rates of morbidity and mortality. Currently, median survival for stage IV melanoma is between 6 and 10 months, a rate that has changed little over the last 30 years.
Vemurafenib (ZELBORAF®; also known as PLX4032) is a small-molecule B-Raf kinase inhibitor for the treatment of cancers harboring activating BRAF mutations. The primary focus of vemurafenib use is in melanoma (>50% harbor activating BRAF mutations). The applications of vemurafenib with other solid tumors, such as colorectal carcinoma (>10% harbor BRAF mutations), are also under investigation. Purified kinase assays have demonstrated that vemurafenib and its related analogs are highly potent inhibitors of B-Raf activity, with 3-fold selectivity for the V600E mutation over the wild-type kinase.
In preclinical models, vemurafenib and its analogs inhibited the growth of B-RafV600E positive melanoma cell lines both in vitro and in vivo. In the initial stage of phase I clinical study, cohorts of patients with advanced solid tumors were treated with escalating doses of vemurafenib (ranging from 200 to 1,600 mg), administered twice-daily (BID) as oral capsules. The crystalline formulation yielded only modest drug exposures, so vemurafenib had to be reformulated as a micro-precipitated bulk powder (MBP), which can increase drug bioavailability.
Vemurafenib have low solubility in water and consequently showed a low dissolution rate and as a result exhibited poor bioavailability, especially when administered orally in crystalline form. Poor bioavailability can cause unpredictable absorption of the compound by the patient, leading to unpredictability in dose/therapy effects. Often food may affect the ability of the patient to absorb such poorly bioavailable compounds, and requiring dosing regimens to be adjusted accordingly. Also, an erratic absorbed drug commands a large safety margin. Moreover, such a drug will require higher doses to achieve sufficiently high systemic or target concentrations for efficacy, which may not be practical in some cases or may leadto adverse effects in others (Testa et al., Prodrugs: bridging pharmacodynamic/pharmacokineticgaps. Curr. Opin. Chem. Biol. 2009, 13, 1-7).
Prodrugs are analogues of active pharmaceutical ingredient that following administration are converted or metabolized to an active form of the drug in vivo. Prodrugs are used to modify one or more aspects of the pharmacokinetics of a drug in a manner that enhances the therapeutic efficacy of a drug. Prodrugs can provide possibilities to overcome various barriers to drug formulation and delivery such as poor aqueous solubility, chemical instability, insufficient oral absorption, rapid pre-systemic metabolism, and toxicity.
Clearly, a significant problem with oral administration of vemurafenib is the bioavailability at larger doses, which are required for the therapeutic efficacy of vemurafenib (approved daily dose is 960 mg BID in MBP formulation). The superior efficacy of vemurafenib against melanoma makes vemurafenib a particular compelling candidate for the preparation of its analogues that can be easily formulated to provide better oral bioavailability. Thus, there is a need for prodrugs of vemurafenib with demonstrated enhanced oral bioavailability. In particular, masking nitrogen in azaindole NH moiety and/or in aryl sulfonamide NH in vemurafenib facilitated absorption throughout gastrointestinal tract. These vemurafenib analogues can enhance the convenience (by reducing the dose and dosing frequency), efficacy, and lessen side effect profile of vemurafenib. In addition, vemurafenib analogues may also be able to be formulated in solution form for intravenous administration or in extensive release forms.